1. Field of the Invention:
The present invention relates to an electromagnetism suppressing material used to block unnecessary radiation produced by an electronic appliance, an electromagnetism suppressing device that uses the electromagnetism suppressing material, and an electronic appliance that is constructed using the electromagnetism suppressing material or the electromagnetism suppressing device.
2. Description of the Related Art:
Increasing use of high-frequency electromagnetic waves in recent years has been accompanied by a new environmental problem of electromagnetic noise causing damage such as the malfunctioning of appliances and adverse effects on the brain and body. As one example, wide use is made of the frequency band 2.45 GHz where wireless communication can be carried out without a license, such as by wireless LANs (IEEE 802.11b Standard), Bluetooth, and ISM (Industrial, Scientific and Medical) appliances. As information appliances are converted to digital processing or use higher clock frequencies, harmonics may occur on this band. Since the number and applicability of both potential sources of electromagnetism and interfered-with devices are increasing exponentially, the risk of interference occurring is increasing astronomically. To counter the problem of electromagnetic interference (EMI), individual appliances need to be prevented from radiating unnecessary electromagnetic waves that can interfere with the normal operation of other appliances (suppression of emission) and also to have sufficient immunity so as to remain unaffected by electromagnetic waves that penetrate from outside (improved immunity). This is referred to as “electromagnetic compatibility” (EMC), and a number of standards for establishing whether electronic appliances exhibit EMC in electromagnetic environments have been determined.
For example, when implementing EMC measures during circuit design, the principle method is to use interference suppression elements as circuit elements for reducing the interfering electromagnetic waves generated from an electronic appliance and to prevent electromagnetic interference from penetrating the electronic appliance. A variety of interference suppression elements exist, such as an LC filter that is a combination of a capacitor and a coil, a varistor or the like. Such elements are designed so that there is little loss when a desired signal passes through the element but a large reflection loss and transmission loss for interference, and they are combined according to an appropriate method and used in most electronic circuits. However, there are cases where the voltage and the current waveform fluctuate due to a particular resonant frequency that depends on how the circuit elements are combined, with this resulting in a large distortion in the desired signal waveform. In addition, the wavelengths of electromagnetic waves in a GHz band are close to the circuit length of an electronic circuit, so that there is the risk of malfunctioning due to the circuit itself acting as an antenna for the electromagnetic waves.
Such EMC problems that cannot be overcome by circuit design have been addressed by a mounting design. As one solution, attention has been focused in recent years on using electromagnetism suppressing members or electromagnetism absorbing members where magnetic powder is mixed with resin and used to form sheets. Such magnetic sheets are mainly used in two ways. In the first, magnetic waves radiated from an antenna are absorbed, while in the second, a sheet is used as a harmonic filter that prevents harmonic noise components from being added at an antenna.
According to the principle of electromagnetic wave absorption, most of the incident electromagnetic energy is converted to heat energy inside an electromagnetism suppressing member or electromagnetism absorbing member. This means that an electromagnetism absorbing member can reduce both the energy reflected at the front surface and the energy transmitted to the rear. The mechanisms for conversion to heat energy are classified as “conductive loss”, “dielectric loss” and “magnetic loss”, with the electromagnetic absorbed energy P per unit volume (W/m3) being expressed in Equation 1 using the electric field E, the magnetic field H, and the frequency f. The first term represents the conductive loss, the second term represents the dielectric loss, and the third term represents the magnetic loss.
                    P        =                                            1              2                        ⁢            σ            ⁢                                                          E                                            2                                +                      π            ⁢                                                  ⁢            f            ⁢                                                  ⁢                          ɛ              ″                        ⁢                                                          E                                            2                                +                      π            ⁢                                                  ⁢            f            ⁢                                                  ⁢                          μ              ″                        ⁢                                                          H                                            2                                                          <                  Equation          ⁢                                          ⁢          1                >            
Conductivity: σ
Complex dielectric constant: ∈=∈′−j∈″
Complex magnetic induction rate: μ=μ′−jμ″
In Equation 1, the first term expresses the conductive loss, the second term expresses the dielectric loss, and the third term expresses the magnetic loss.
Also, in addition to the electromagnetism absorbing effect described above, an electromagnetism suppressing member or an electromagnetism absorbing member used on a transmission path is thought to have a filter effect due to the inductor component that is particular to the magnetic material.
At present, magnetic sheets that are used as an electromagnetism suppressing member or an electromagnetism absorbing member are stuck onto printed circuit boards, flexible printed circuits (FPCs), the rear surface of a case, the upper surface of a package, and the like. However, due to the complex signal transmission paths inside electronic appliances and the differences in electric and magnetic field distributions produced by such, it is presently difficult to clearly establish the principles and effects of such electromagnetism suppressing members and electromagnetism absorbing members. Also, since the characteristics and effects of magnetic sheets are not clearly understood, functional evaluations of magnetic sheets are often carried out by estimating using the harmonic filter effect (suppressing effect) mainly on a microstrip line and the magnitudes of the losses (∈″, μ″).
An electromagnetism absorbing material where minute particles of ferromagnetic material are uniformly dispersed in a medium such as water, glycerine, or methyl alcohol as a medium with a high dielectric constant has been disclosed (see Patent Document 1).
<Patent Document 1>
Japanese Laid-Open Patent Publication No. 64-52302
However, when a magnetic sheet is stuck onto a printed circuit board, and in particular onto an IC package, or onto the rear surface of a housing to block unnecessary radiation in an electronic appliance as described above, it is often not possible to achieve the expected electromagnetism absorbing effect, and conversely a phenomenon where the magnetic field strength actually increases may be observed. To remedy this situation, an electromagnetism absorbing sheet with better absorption characteristics than existing magnetic sheets is required. With existing sheets, if the included amount of magnetic powder is increased to strengthen such characteristics, the specific gravity of the magnetic sheet will rise together with the amount of magnetic powder, resulting in the magnetic sheet becoming hard. When attached to a flexible printed circuit (FPC), an electromagnetism absorbing sheet needs to be more flexible so that the flexibility of the FPC can be utilized. In addition, the use of magnetic powder results in high costs, and a less expensive electromagnetism absorbing sheet is required.